The invention relates to invisible inks that can form inverse-contrast, machine-readable images using conventional ink jet printers and that are useful in a process that enables enhanced readability of Information-Based Indicia (IBI) images especially on traditionally low-contrast papers.
A wide variety of paper types is available for postal uses, but the variation in available colors and inks can cause readability problems when printing with visible inks for both manual and machine reading. While those skilled in the postal arts are aware that some color combinations can decrease print contrast to marginal levels, the reliability of routine mailing is largely subject to color and paper selection made for reasons of taste and economy. It would be desirable to have an ink and a process that would permit machine readability largely independent of paper color.
Typically, reflectance of printed images is the primary basis for both visual and machine readability. In other words, when visible light hits an address block on an envelope, either a human or a machine attempts to read the address information based on the reflectance of light—typically a dark print image reflecting less light than a light-colored envelope. In these systems, contrast between printed and unprinted areas will determine readability. Because there is a wide variability in envelope color, the use of reflectance can never be ideal. It would be desirable to have an ink and a process that provided better readability over a wide range of paper colors.
Information-Based Indicia (IBI) images are becoming increasingly important for security and address information purposes. IBI images are composites comprised of 2-D bar code information, typically comprising postage information, and human-readable postage information, which typically includes address information. The images can be printed utilizing open systems using conventional ink jet printers. Reliability is a major concern, and there is a need to improve the reliability of machine reading these images.
Invisible inks have been utilized to some extent in the past for printing machine-readable 2-D bar codes, but inks as typically employed often possess excitation and emission spectra that overlap with optical brighteners (and other sources of fluorescence indigenous to a particular system) used in envelopes and for other practical purposes. Moreover, printing of 2-D bar code IBI images with invisible inks has been less than refined because available inks typically have poorly defined excitation and emission spectra and are not water-based suitable for inkjet printing. The available inks do not function well in providing highly-reproducible prints for reverse contrast reading utilizing short wavelength ultraviolet (UV) light for excitation causing fluorescence in the visible range, e.g., green and red through infrared regions. While some systems employ invisible inks in combination with inks visible to the human eye, including for printing 2-D bar code IBI images, few are available for producing reliably readable images by ink jet printing.
Some prior art procedures use invisible inks along with visible inks. For example, in U.S. Pat. No. 5,502,304, Berson, et al., describe a system that employs upper and lower bar codes in a format that provided a degree of security. The lower layer bar code is written on an object with a visible dark ink and an upper layer bar code is written over the lower layer bar code with an ink that is invisible to the naked eye. A detector apparatus is described for reading both bar codes in a manner that detects authenticity. And, in U.S. Pat. No. 5,525,798, Berson, et al., describe inks that are selectively excitable by different wavelengths of incident radiation. The invisible inks in both cases, however, had relatively broad ranges of excitation and emission.
In U.S. Pat. No. 5,693,693, to Auslander, et al., wax-based invisible inks are described that emit light at various wavelengths in the visible region when they are excited by UV light. This allows lower layer clear text information to be written on an object with a regular, visible ink and an upper layer bar code to be written over the lower layer text information with an ink that is invisible to the naked eye. In this manner, more information can be provided than with conventional bar codes. However, the inks were not water-soluble. In U.S. Pat. No. 5,684,069, to Auslander, an invisible ink is described that utilizes a substituted phthalocyanine dye in combination with selected waxes and resins. The ink is useful in thermal printing, not ink jet printing and is responsive to infra-red light in the range of 720 to 1000 nm.
Inks that are selectively excitable by different wavelengths of incident radiation are described by Auslander, et al., in U.S. Pat. No. 5,542,971. The use of these inks allows a lower layer bar code to be written on an object with a dark visible ink and an upper layer bar code to be written over it with an invisible ink. In this manner, the lower layer and upper layer bar code can contain more information than conventional bar codes. The invisible inks used are based on complexes of rare earth elements with an atomic number higher than 57 such as: Eu, Gd, Tb, Sm, Dy, Lu with various chelating agents providing chromophore ligands that absorb in the ultraviolet and the blue region of the spectra such as: β-diketones, dipicolinic acid, etc.
Yet other invisible fluorescent jet inks are described in U.S. Pat. No. 5,837,042, to Lent, et al. The inks are said to be suitable for producing security markings. The jet ink compositions comprise a fluorescent colorant, an ink carrier, and optionally one or more binder resins. The markings are invisible to the unaided eye and are visible only when excited by ultraviolet light. The colorant comprises a rare earth metal and a chelating ligand, is excitable by ultraviolet light having a wavelength of from about 275 nm to about 400 nm, and fluoresces at a wavelength of from about 550 nm to about 700 nm. The chelates can comprise a rare earth metal such as europium, dysprosium, or terbium.
U.S. Pat. No. 6,402,986, to Jones, II, et al., describes compositions said to be different from those of Lent, et al. The compositions are used in methods for the verification of products or documents based on the reading of emitted light from luminescent compositions that can be incorporated or applied to a wide variety of materials. The compositions are luminescent at various wavelengths, displaying qualities of image, wavelength, and time scale for the measure of luminesce decay. The light emitted from the composition, and specifically, the measure of the variable and adjustable luminescence lifetimes provides a multi-parameter signature for purposes of comparative light decay analysis of verification marks or features. They note that the 1,3-diketone ligands of Lent, et al., do not show long term stability to light, and describe other compositions based on metal chelates wherein three ligands are bound to the metal.
In U.S. Pat. No. 6,149,719, Houle describes light-sensitive invisible ink compositions and methods for using them. The disclosed system generates high-definition, lightfast images that are easily read or otherwise detected using far red, infrared, and/or ultraviolet light. The inks contain an uncomplexed invisible metal phthalocyanine far red/infrared fluorophore (optimally chloroaluminum[III]phthalocyanine tetrasulfonic acid or salts thereof). An ultraviolet fluorophore can also be included. The inks are invisible to the unaided eye, but when exposed to far red or infrared light (wavelength=about 650-715 nm) they fluoresce at a wavelength of about 670-720 nm. When an ultraviolet fluorophore is employed, the inks can also be detected by applying ultraviolet light (wavelength=about 250-380 nm) which results in fluorescent emission at a wavelength of about 400-650 nm. The inks are said suited for forming invisible images using inkjet technology. In U.S. Pat. No. 6,458,294, Oshima, et al., describe a fluorescent substance useful in ink jet inks containing both an Ln composition, which includes at least one element selected from the group consisting of Nd, Yb and Er and another element selected from the group consisting of Y, La, Gd, Bi, Ce, Lu, In and Tb at a ratio within the range of 0.01 to 0.99.
There remains a need for water-based invisible inks having narrow excitation and emission spectra that can produce highly-reliable inverse-contrast, machine-readable IBI images using conventional ink jet printers.